6th International Conference of Applications of Stable Isotope Techniques to Ecological Studies

Event date :

August 2008

Event place (city) :

Honolulu

Event country :

Hawai'i

Abstract :

[en] Metabolic activity is positively related to temperature, inversely to body mass and is a function of taxon specific life style features, in particular activity such as level of active movement. Therefore, the isotope signal transfer velocity is expected to be lower in cold environments and in larger as well as less active organisms.
Our study explores whether this may be a problem in trophic studies of a comparatively “slow” because cold system such as the high Antarctic shelf ecosystem and in comparatively large organisms such as benthic amphipod species within this system.
We compare experimentally the velocity of stable isotope signal transfer from prey to consumer in three lysianassoid amphipods, Waldeckia obesa, Abyssorchomene plebs and Pseudorchomene coatsi. They have similar alimentation, but different size and lifestyle. Indeed, W. obesa is a very sedentary species spending most of the time immobilized on diverse substrates whereas P. coatsi is very motile, swimming rapidly around the aquarium. The third species, A. plebs has an intermediate behaviour, sharing time between short swim and resting on bottom. Those species also differ significantly in size: and are good representative of scavenger trophic guild on Antarctic shelf.
After being starved, amphipods were kept by species and fed ad libitum with lyophilized fish during fifty days. Individuals were sacrificed weekly for isotopic analysis. At the end of the 7-week incubation with standardized food, rank correlation of δ13C and δ15N against time did not show any consistent trend for A. plebs (δ13C: p = 0.51 and δ15N p = 0.04) neither for the species W. obesa (δ13C: p = 0.77 and δ15N p = 0.26). By contrast, for P. coatsi, rank correlations were highly significant (p < 0.0001). The linear regression illustrated a clear increase of isotopic ratios all along the experiment. This metabolic discrepancy between species is probably a size-mass effect. Furthermore, for this species, ANCOVA of the individually measured isotopic ratios first transformed to an offset value (rate vs carbon ↔ nitrogen, covariate time) provided evidence for significant effects of the parameter “isotope” on isotopic temporal evolution. Indeed, the δ13C values evolve much faster than the δ15N ones. According to data, it would take double time for P. coatsi to balance its nitrogen isotopic signature than its carbon isotopic ratio when changing food. Those results are critically discussed.